The present invention relates to machine tools in general, and more particularly to improvements in machine tools wherein one or more parts are movable up and down by displacing units of the type employing an upright feed screw and a spindle nut which meshes with the feed screw. Typical examples of such machine tools are many surface grinding machines wherein one or more tool carriers are movable up and down in response to rotation of the feed screw relative to the nut or vice versa.
Displacing units which employ a feed screw and a mating nut can be used with advantage in many types of machine tools, i.e., not only in surface grinding machines. In many instances, the feed screw is designed to move axially, i.e., up and down, and to share its movements with a carrier for one or more tools. If the machine tool is designed to treat the workpieces with a very high degree of precision, the feed screw is rotated in spherical or otherwise configurated rolling elements so as to reduce the extent of stray movement when the feed screw is caused to move up or down. As a rule, the means for driving the feed screw or the nut includes a motor and a suitable transmission, for example, a torque transmitting unit which employs several toothed pulleys or gears and an endless toothed belt trained over the pulleys. If the displacing unit is not provided with any self-locking means, breaking of the toothed belt can entail uncontrolled and rapid descent of the feed screw with the part or parts which are secured thereto. This can cause serious damage to the machine, lengthy interruptions in operation, as well as injury to the attendants. Uncontrolled descent of vertically movable parts is particularly dangerous in many types of grinding machines wherein the feed screw carries one or more carriers for grinding wheels which can be destroyed when the belt breaks and the feed screw descends at an excessive speed. Destruction of the grinding wheel or wheels can cause serious injuries.